Nanocapsules and microcapsules are useful for encapsulating a variety of functional core contents, such as indicating agents, healing agents, corrosion inhibitors, pharmaceutical drugs, food additives, and paints. Blaiszik et al., Annu. Rev. Mater. Res. 2010, 40, 179-211; White et al., Nature 2001, 409, 794-797; Esser-Kahn et al., Macromolecules 2011, 44, 5539-5553. Many of the capsules have a solid polymeric shell wall and encapsulate a functional liquid agent core. The capsules are designed to perform when they are exposed to certain intended stimuli, which rupture or disintegrate the shell wall of the capsule and releases the functional agent contained therein to fulfill its function. Jones et al., Compos. Sci. Technol. 2013, 79, 1-7; Cho et al., Adv. Mater. 2009, 21, 645-649; Yuan et al., Macromolecules 2008, 41, 5197-5202.
In many applications, the capsules may be stored or used for a long period of time before being needed to perform their intended function. Patel et al., Composites Part A: Appl. Sci. Manuf. 2010, 41, 360-368. The capsules may also be exposed to severe conditions that could potentially compromise the integrity of the capsules and cause premature rupturing of the shell wall and release of the core content. Any loss of core content prior to intended stimuli exposure, such as during storage, manufacturing, or service, can lead to reduced beneficial effects of the encapsulated systems in their application. Jin et al., Polymer 2012, 53, 581-587. When capsules are exposed to harsh environments, such as elevated temperature or strong solvents, the capsule shell wall may degrade and accelerate diffusion of the core content due to chemical gradients. In self-healing polymers, high processing temperatures (>100° C.) may cause core diffusion to the host matrix, reducing self-healing capability. Jin et al., Adv. Mater. 2014, 26, 282-287. Additionally, wet processing in polymeric composite fabrication often involves strong solvents that can degrade the capsule shell wall. Yuan et al., Mater. Chem. Phys. 2008, 110, 417-425.
The stability of poly(urea formaldehyde) (UF) shell wall microcapsules at different temperatures has been studied. Yuan et al., J. Mater. Sci. 2007, 42, 4390-4397. This article shows that microcapsules start to dimple at 120° C. for 2 hours due to the diffusion of the liquid core content (a mixture of butyl glycidyl ether and diglycidyl ether of bisphenol A) from the microcapsule shell wall. Diffusion of core content was also observed when the microcapsules were immersed in acetone solvent. Double layered core-shell microcapsules have been prepared by condensing urea-formaldehyde (UF) resin on as-prepared polyurethane (PU) microcapsule surfaces. Li et al., Polym. Bull. 2008, 60, 725-731. Subsequently, an improved single-batch process has been described for the preparation of PU/UF double layered microcapsules by adding PU prepolymer into the liquid core phase and allowing PU interfacial polymerization for inner PU shell wall formation at the same time as outer UF formation. Caruso et al., ACS Appl. Mater. Interfaces 2010, 2, 1195-1199. Though increased thermal stability has been observed with this process, the microcapsules still exhibited moderate core content loss at elevated temperatures (˜10 wt % loss at 180° C. for 2 h). Furthermore, the stability of the microcapsules in harsh environments (e.g., in strong solvents) was not evaluated in this article.
In self-healing polymer applications, small size-scale self-healing of sub-micron crack sizes has motivated the realization of sub-micron microcapsules. As the size of microcapsules decrease, the microcapsule shell wall thickness also decreases (from ca. 300 nm of 250 μm diameter microcapsules to ca. 50 nm of ca. 2 μm diameter microcapsules). Therefore, the stability of small size microcapsules becomes a concern. Another study reported a technique to coat a 20-40 nm thick silica layer on microcapsule surfaces for improved environmental stability. Jackson et al., Macromol. Rapid Commun. 2011, 32, 82-87. To address the fragility of small size microcapsules, this study developed a silica coating technique to coat 20-40 nm thick silica onto microcapsule surfaces and dispersed the microcapsules in an epoxy matrix. However, two problems were observed with this process—relatively poor interfacial adhesion between the inorganic silica and the organic epoxy matrix and an increased stiffness of the microcapsules imparted by the silica coating. In self-healing applications, the low adhesion strength and high stiffness may lead to deflection of the crack from the microcapsules instead of rupturing the microcapsules as the crack propagates. Thus, the silica coated capsules may hinder or prevent the release of the functional core content when damage occurs.
Polydopamine (PDA) is an important polymer formed by the oxidation of dopamine. It is commonly used for coating various surfaces and recently has been applied in the biomedical field, for example, from coatings for interfacing with cells, to drug delivery and biosensing. Lynge et al., Nanoscale, 2011, 3, 4916-4928. The coexistence of catechol and amine groups found in proteins of mussels is believed to provide strong adhesion properties. Dopamine is a building block that contains both catechol and amine moieties. As a result, the polymeric form of dopamine, PDA, shows strong adhesion to many types of surfaces. Lee et al., Science 2007, 318, 426-430; Ryou et al., Adv. Mater. 2013, 25, 1571-1576. Under basic conditions, dopamine immediately undergoes polymerization and deposits on the target surface as PDA. Dreyer et al., Chem. Sci. 2013, 4, 3796. Though the detailed structure of PDA remains elusive, recent investigations have indicated that hydrogen bonding and π-π stacking make PDA a dense membrane with remarkable stability. Dreyer et al., Langmuir 2012, 28, 6428-6435; Liebscher et al., Langmuir 2013, 29 (33), 10539-10548.
Bio-compatible and superparamagnetic Fe3IO4/Polydopamine nanocomposites have been synthesized for use in catalyst supports and drug delivery. Si et al., Materials Chemistry And Physics, 2011, vol. 128, no. 3. Chinese patent application number CN102861921A describes a method of synthesizing magnetic/gold nanoparticles with Fe3O4/PDA for use in drug delivery. Chinese patent application number CN101966441A describes a method of synthesizing microcapsules with calcium carbonate particles and PDA, then removing the calcium carbonate to provide a hollow microcapsule. US patent application publication number US 2012/0237605A describes nanoparticles with a gold, silver or iron oxide metallic core and a PDA coating. The loading and release behavior of PDA capsules have also been evaluated. Yu et al., Chem. Commun., 2009, 6789-6791.
Notwithstanding the advances in the capsule field, there remains a need to prepare improved capsules that are stable and robust. Given the sophistication of recent advances being utilized in capsule technology and the growing trend to utilize capsules, this need is more urgent than ever. Accordingly, there is a need for a more stable capsule that can withstand a wide variety of manufacturing, processing, servicing and environmental conditions. In particular, there is a need for environmentally stable capsules containing liquid functional agents. An optimum capsule would be stable over a wide spectrum of environmental conditions and be able to store the core content over an extended period before a triggering mechanism is activated.